is it possible to add a single passive sub to a 2 channel valve amp set-up?

The reason I ask is that I have been given a passive sub box from an LG home entertainment system, I have a 2 channel sweet peach 10w valve amp, and wondered if I could hook it up reasonably easily (guessing some kind of crossover needed) to relieve my small speakers of the job of reproducing the bass elements. They really suffer from bass distortion above quite small volumes.

It is possible, but not easy (or cheap) to get it right, a low frequency passive crossover requires very large coils and capacitors, and the sub must be more sensitive (which means BIG) than the top cabinets, or power wasting resisters must be added to attenuate the top cabinets.

It is generally less money and better outcome to simply get a plate amp for the sub.

I was fairly certain it wouldn't be a simple task but thought it worth asking.

In reality, yes, the speakers are a little small (woofer is 3.5"). Although rated for 75w they have problems when playing a bass heavy track at anything more than moderate volumes as the woofers just move too much causing distortion. At lower volumes they are fine, good detail etc.

I've built two budget systems for friends fairly recently. One a full system
with a used Pioneer A-300X amplifier, and the other an I-pod system with
a dock and a used original Rega Brio amplifier. The speakers I chose are :

At £100 a pair they take up the space of a small speaker on a stand for
less than the cost of of a good pair stands for a good small speaker.

I don't know what speakers you have, but I told my friends of course
better speakers are available, (for a lot more money), but you should
get used to decent speakers (very decent for £100), before going out
and auditioning / choosing more expensive speakers to their taste.

My personal opinion is the CR30.4's are outstandingly good for £100, and
anyone who buys them will have no problem moving them on to friends
or family, to such an extent that they are a better choice than £100 used.

FWIW both my friends are more than happy with them and have no plans
at all to upgrade them. They both say their expectations were exceeded.